Mar 20 2020

Are Artificial Leaves Plausible?

We have many current solutions to the energy challenges we face – solar, wind, geothermal, hydroelectric, nuclear. I and others have previously argued that if we are going to have the best chance to minimize climate change as much as possible, we need every option on the table. That is partly because no one of them is perfect, and they get worse the more you try to push penetration into the energy infrastructure. So best to pick the low-hanging fruit from each. Also, if we try to solve our energy problems with one solution we would likely run into shortages of raw material and optimal locations.

In fact, it would be nice to add more options. Researchers are aggressively trying to work out the challenge of fusion energy, for example. Even if that succeeds it won’t be the one optimal solution for all our energy needs, but it will help a lot. We also need better grid storage options. You may also notice that I did not have biofuels on the list, because I think it’s unlikely they will make a major contribution, at least not if they need to use up arable land that could be used for food production. But if we can develop a biofuel process that does not require premium arable land, that could fill a niche also.

There is another possible technology on the horizon that occasionally grabs headlines when some incremental advance is made – the artificial leaf. This term is used to refer to any process that uses light in a photosynthesis type process. So it is not a photovoltaic, directly creating electricity, but rather is using light to split water and combining the hydrogen with CO2 from the atmosphere to make fuel. Essentially, this is artificial biofuel. Then why not just use biological leaves to make biofuel? That gets back to the land issue – you could theoretically have a biofuel plant using an artificial leaf process in the desert. I suppose theoretically you could also use salt water for the process, and would therefore not be a drain on our fresh water supply (but I am not sure about this).

Researchers at Department of Energy’s Lawrence Berkeley National Laboratory recently published a study showing that they have demonstrated all the necessary steps in an artificial leaf process. The next step in their research is to put it all together as a functioning artificial leaf. Here is how it works:

“Each tiny (about 0.5 micrometer wide), hollow tube inside the tile is made of three layers: an inner layer of cobalt oxide, a middle layer of silica, and an outer layer of titanium dioxide. In the inner layer of the tube, energy from sunlight delivered to the cobalt oxide splits water (in the form of moist air that flows through the inside of each tube), producing free protons and oxygen.”

The free protons then combine with CO2 to form carbon monoxide, which is a high energy feed molecule for fuel. They hope to alter the process so that it directly produces methanol, which would save a step in producing usable fuel. That is one of the challenges they face. They other is scalability (always a thorny issue). They are planning on building small tiles several inches on a side for testing.  If this works they will need to assemble these small tiles into huge farms capable of producing massive amounts of fuel. This is not a given – scalability is often an issue that kills otherwise promising laboratory processes.

The current advance is just one small step in the process, but we now have a proof-of-concept for every step in the artificial leaf. That is always a good position to be in, knowing that a technology is theoretically possible. But that is not enough, and it remains to be seen if the process will be feasible on an industrial scale.

One major concern I have is the material, specifically the cobalt. We are already having major concerns about the supply chain of cobalt which is used in current lithium ion batteries. In fact researchers are trying to redesign the battery to use more abundant materials. Cobalt is mostly mined in the DRC and refined in China. The DRC is not exactly a stable part of the world. Introducing a new technology that will also require massive amounts of cobalt is probably a non-starter. This again gets back to my point that we don’t want to have to scale up any one energy source too much, because then we start to run into supply problems for raw material. I don’t know if the researchers are prioritizing replacing cobalt with another material. At this stage they are probably just trying to get it to work, and then they will worry about industrial scalability later. But this is why so many lab processes go nowhere.

There is always a balance between basic and applied science, and so figuring out how to make an artificial leaf would be a great accomplishment even if the tech is not feasible or cost-effective as is. But the clock is also ticking and we need new energy tech now. I suspect the researchers all know this very well, but one step at a time. This is partly why advances in this kind of technology take so long – in order to make it all the way to an effective energy solution, the technology needs to check a lot of boxes. Using cheap, available, and environmentally friendly material is just one, but it’s critical.

I have been following the artificial leaf news for a while and so far there have been many small incremental advances, and we are getting to the point where all the theoretically pieces are in place. But still, it could take a decade or two to develop a functioning technology. My sense is we are where we were with hydrogen fuel cell technology 20 years ago. But these things are also somewhat unpredictable. A lab may announce a breakthrough tomorrow that will change the game. But if I were playing the odds I would say that a workable artificial leaf technology is at least a decade away. We therefore have to ask – where will we be in a decade? If all electric vehicles replace gas vehicles sufficiently, the market for biofuel may be significantly reduced before the tech can get off the ground. I suspect there will always be a niche market, for large vehicles and aircraft, and that may be where producing artificial fuel remains necessary.

There are still lots of possibilities, and it would be very interesting to see what our energy infrastructure will look like in 20 years.


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